1. Field of the Invention
This invention pertains broadly to the field of communications. More particularly, the invention pertains to Doppler compensations systems. In greater particularity, but without limitation thereto, the invention relates to a Doppler compensation system that provides precision Doppler correction to acoustic signals so that these signals may be readily utilized with high performance telephonic modems.
2. Description of the Prior Art
In a continuing effort to advance the state of underwater communications, acoustic engineers have sought to utilize high performance modems such as those commonly used in conjunction with telephonic communications. The great expenditures made by the industrial sector in perfecting these modems have made them an attractive alternative to producing yet another set of modems uniquely tailored for acoustic communications.
Modems developed for telephonic usage operate upon communications that travel at the speed of light, therefore they are designed with the expectation that the communications will experience negligible Doppler effects. As a result, these modems are typically designed to tolerate very slight frequency errors. Further, these high performance modems are usually not designed to tolerate a high degree of data rate error. In either case, if frequency or data rate errors are high, the modems suffer serious performance degradation.
Acoustic communications are particularly susceptible to effects in frequency and data rate due to the Doppler effect. This is due in part to changes in the orientation of an acoustic transmitter with respect to an acoustic receiver. It is typical for an acoustic transmitter to move with respect to an acoustic receiver or for the receiver to move with respect to the transmitter or for a combination of these. Because of the possibility of transmitters and receivers moving towards or away from each other at various rates, transmissions under these conditions can experience Doppler shifts varying both in magnitude and direction.
The effect of these movements upon transmission frequencies and data rates can be more readily understood by examining the effects upon one cycle of a transmitted sine wave. In the instance where a receiver moves away from a transmitter with the velocity v, the receiver in one cycle of a sine wave of frequency f will move v/f. This, in turn, makes the received wavelength v/f longer than the transmitted wavelength, which is c/f where c is the speed of sound in water. The received wavelength is thus (c+v)/f and the received frequency is thus: EQU [c/(c+v)].multidot.f=(1-(v/c)-(v/c).sup.2 -. . .).multidot.f
Since c=4900 feet/second, which is much greater than v (in knots), EQU c/(c+v).apprxeq.1-v/c=1-0.00034
This amounts to a received frequency change of approximately 0.034 percent of the transmitted frequency per knot of velocity between the transmitter and receiver.
For example, an acoustic transmission from the output of a commercial modem through water on a representative 11.33 kilohertz sideband modulated carrier, if uncorrected, will result in a received frequency error of approximately (11.33 kilohertz) (0.034 per cent per knot)=3.85 hertz/knot at the receive commercial modem inputs.
High performance commercial modems typically used with telephone systems may tolerate some frequency differences between transmissions and receptions, however these modems can typically tolerate no more than 7 to 10 hertz of frequency errors without serious system downgrading. Yet frequency shifts of greater than 7 hertz are frequently encountered between acoustic transmissions and receptions, where differences in the velocity of receivers with respect to transmitters are often greater than 1.8 knots, at 3.85 hertz per knot frequency error.
Related to the shift in frequency is a corresponding shift in data rate. As previously discussed, the received data rate differs from the transmitted data rate by 0.034 per cent per velocity (knot) between the transmitter and receiver. As high performance modems operate with communications travelling at the speed of light and exhibiting very little Doppler effect, they are designed to accommodate very slight data rate shifts, typically on the order of no more than 0.02 per cent of received data rate error.
There exists a variety of methods and means for correcting Doppler effect affecting transmitted signals. One technique sends upper and lower reference frequencies enclosing an information band of frequencies. The difference between the reference frequencies, as Doppler effected, are multiplied by a selected multiplying factor so as to equal a predetermined constant. This technique is limited by the number of available multiplying factors and provides merely a rough correction for Doppler effect.
Another Doppler compensation means correlates a set of electrically Doppler effected replicas to a transmitted signal. A high correlation peak corresponds to the velocity and range of a moving target. As with the first described method this method is limited by a finite number of replicas available, resulting in a Doppler correction of less than desirable accuracy for some applications.
In yet another prior art scheme, a reference tone is transmitted along with information channels. The shift in the reference tone due to the Doppler effect is corrected accurately, and this correction is applied to information frequencies on either side of the reference frequency. Where the information frequencies occupy a relatively narrow band, the Doppler shift affecting the frequencies would be approximately the same as that affecting the reference tone, and the correction would be relatively accurate. Yet the more the information frequencies deviate from the reference tone, the more the correction becomes less and less precise. For some applications, this correction would be within the tolerances of the system's receiver.
While the above described prior art methods have proven to be satisfactory for their intended purposes, application of these methods to acoustic communications made by way of high performance modems would provide system performance that could be considered less than desirable.